Abstract

Organic-rich shales (ORS) are common source rocks for most clastic reservoirs. More recently, they have gained importance as reservoirs. However, the processes of kerogen maturation, hydrocarbon storage, and hydrocarbon transport are still poorly understood. Empirical relations have been developed to relate the increase in acoustic velocity and elastic modulus with increasing maturity. The reason for this increase in velocity also remains poorly understood.

We conducted experiments on ORS samples with a range of maturities from the Bakken formation. Our study focuses on investigating methods of predicting maturity of ORS by evaluation of their impedance micro-structure. Our Young’s modulus measurements on a nanometer scale help understand variations of Young’s modulus of minerals, clay particles, and kerogen matter in naturally matured shales. The samples were re-measured after subjecting them to hydrous pyrolysis. This step helped us investigate the cause for change in modulus with maturity and the mobility of the pyrolysed organic matter.

In the naturally matured samples, we find direct qualitative relationships between the Young’s modulus of shale samples and its maturity indicators, such as TOC and Transformation Ratio. After hydrous pyrolysis, there is a significant lowering of the Young’s modulus in some immature samples. We will present results of elastic property changes before and after hydrous pyrolysis in shales of various maturities.

This study improves our current understanding of maturity-related variations by using analysis from nanoindentation. We integrate these measurements with geochemical analysis, and observations from downhole sonic measurements to develop relationships of elastic impedance to shale maturity. These results are critical to help us understand how shales evolve with burial and maturation and how hydrocarbons are stored and transported to sustain large storage even at high overburden stresses.

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